1. Field of the Invention
The present invention relates generally to methods and apparatus for assembling semiconductor dice to a carrier substrate. In particular, the present invention relates to methods and apparatus of underfill bonding semiconductor dice to a carrier substrate and various assembly arrangements with respect to underfill bonding semiconductor dice to a carrier substrate followed by encapsulation.
2. State of the Art
Chip-On-Board (“COB”) or Board-On-Chip (“BOC”) technology is used to attach a semiconductor die directly to a carrier substrate, such as an interposer or printed circuit board. Electrical and mechanical interconnection used in COB or BOC technology may include flip-chip attachment techniques, wire bonding techniques, or tape automated bonding (“TAB”) techniques.
Flip-chip attachment generally includes electrically and mechanically attaching a semiconductor die by its active surface to a carrier substrate using a pattern of discrete conductive elements therebetween. The discrete conductive elements are generally disposed on the active surface of the die or an interposer during fabrication of the semiconductor die package, but may instead be disposed on the carrier substrate. The discrete conductive elements may comprise minute conductive bumps, balls or columns of various configurations. Each discrete conductive element is placed corresponding to mutually aligned locations of bond pads (or other I/O locations) on the semiconductor die (or interposer) and terminals on the carrier substrate when the two components are superimposed. The semiconductor die is thus electrically and mechanically connected to the carrier substrate by, for example, reflowing conductive bumps of solder or curing conductive or conductor-filled epoxy bumps. A dielectric underfill may then be disposed between the die and the carrier substrate and around the discrete conductive elements for environmental protection and to enhance the mechanical attachment of the die to the carrier substrate. For example, U.S. Pat. No. 5,710,071 to Beddingfield et al. discloses an exemplary flip-chip attachment of a semiconductor die to a substrate and a method of underfilling a gap between the semiconductor die and substrate.
Wire bonding and TAB attachment techniques generally begin with attaching a semiconductor die by its back side or its active surface to the surface of a carrier substrate with an appropriate adhesive, such as an epoxy or silver solder, a liquid or gel adhesive, a double-sided adhesive-coated tape segment such as Kapton®, a polyimide. In wire bonding, fine wires of gold, aluminum or alloys thereof, are discretely attached to bond pads on the semiconductor die and then extended and bonded to corresponding terminal pads on the carrier substrate. A dielectric encapsulant such as a silicone or epoxy may then be applied to protect the fine wires and bond sites. In TAB attachment, ends of metal traces carried on a flexible insulating tape such as a polyimide are attached, as by thermocompression bonding, directly to the bond pads on the semiconductor die and corresponding terminal pads on the carrier substrate.
Particularly in the case of wire bonding followed by a transfer or other molding process to encapsulate a die and carrier substrate assembly, there are problems in securing the semiconductor dice to the carrier substrates using an adhesive-coated tape. Specifically, by conventionally utilizing adhesive tape in attaching a semiconductor die to a carrier substrate followed by overmolding, moisture associated with the adhesive becomes trapped, ultimately resulting in moisture sensitivity issues in the form of enhanced potential for delamination of the components of the semiconductor die assembly. Further, the cost of the large volume of adhesive tape used to attach large numbers of dice to carrier substrates becomes excessive. In addition, the conventional use of substantial volumes (as measured by surface area) of tape is required to avoid stress defect failure in semiconductor die assemblies. Finally, even with the use of substantial tape coverage between a semiconductor die and its carrier substrate, the bond and resulting assembly may be undesirably flexible and resilient.
Another ongoing problem with the use of wire bonding in packaging occurs during a transfer molding encapsulation process of the semiconductor die in what is known as “wire sweep.” Wire sweep results when a wave front of dielectric (commonly a silicon-filled polymer) encapsulation material moving through a mold cavity across the semiconductor die and carrier substrate assembly forces wire bonds to contact adjacent wire bonds and become fixedly molded in such a contacted position after the encapsulation material sets. When wire sweep occurs, a wire bond interconnection of a semiconductor die to a carrier substrate short circuits, which results in a nonfunctional semiconductor die assembly. Wire bond sweeping may also result in bond wire breakage or disconnection from a bond pad or terminal.
Yet another problem with conventional techniques is that of bleed of molding compound introduced into a mold cavity to form a dielectric encapsulant over the die and carrier substrate, which problem particularly manifests itself in the case of BOC-type assemblies wherein bond pads of a semiconductor die accessed through a slot in a carrier substrate are wire bonded prior to encapsulation. Under certain conditions, such as where the die fails to overlap the slot sufficiently, pressure of the molding compound in conjunction with the configuration of the assembly causes molding compound to bleed out of the mold cavity.
Therefore, it would be advantageous to utilize wire bonding in packaging in combination with an assembly and encapsulation technique to substantially eliminate moisture sensitivity issues as well as being cost efficient and providing a more robust semiconductor die assembly. It would also be advantageous to utilize wire bonding packaging techniques while substantially eliminating the problem of wire sweep and molding compound bleed.